The present invention is directed to a differential amplifier.
A prior art differential amplifier is disclosed in the book "Design of MOS VLSI Circuits for Telecommunications" by Y. Tsividis and P. Antognetti, Prentice Hall, New Jersey (1985), pages 129 through 136, particularly FIG. 5.4. Therein, two additional field effect transistors connected in parallel to a power source and serving as controllable power sources are affected by current-mirroring circuits and by the currents flowing in the parallel circuit branches such that one of the additional field effect transistor supplies an auxiliary current that intensifies the quiescent current of the power source when a fixed voltage value is exceeded by an input signal superimposed on a gate bias. The other, additional field effect transistor supplies an auxiliary current that intensifies the quiescent current of the power source when the fixed voltage value is downwardly transgressed by the superimposed input signal. The current-mirroring circuits are structured such that the low quiescent current is greatly intensified by the signal-dependent appearance of the auxiliary currents. Good driver properties for simultaneous reduction of the dissipated power of the differential amplifier are thus achieved.
One disadvantage of this known amplifier, however, is that instabilities that very frequently lead to a distortion of the output signal appear in the control circuits formed by the parallel circuit branches, the current-mirroring circuits and the controllable power sources. These instabilities appear particularly for supply of an input signal having steeply ascending and descending signal edges, for example a square wave voltage.
An earlier German Patent Application P No. 37 01 791.8 discloses a differential amplifier wherein two field effect transistors connected parallel to the power source and serving as controllable power sources are driven from the circuit input via a level-converting circuit such that the low quiescent current is greatly intensified by an appearance of the auxiliary currents dependent on the input signal. Compared to the former, known differential amplifier, the critical advantage herein is that no control of the additional power sources occurs from the amplifier output side. Rather, a control thereof occurs from the amplifier input side, so that no significant distortions of the output signal appear even for square wave voltages to be transmitted.